Recently, from viewpoint of improvement of fuel economy accompanying weight reduction and reduction of exhaust gas of automobiles and the like, further higher strengthening of a steel is required, and with regard, particularly, to a cold rolled steel sheet, hi-ten-ization (high strengthening) has been progressing rapidly. To effect high strengthening by adding alloy elements is common against such a requirement, however, there comes up a problem that chemical conversion treatment property lowers if adding amount of alloy elements is increased. Mo, among them, is commonly used as an element to increase the strength because ductility reduction is little although it has a high effect in increasing the strength. However, if Mo is added to steel, a new problem comes up that natural potential of the steel sheet in the chemical conversion treatment liquid gains to noble direction and chemical conversion treatment property is extremely deteriorated.
So, several methods are also proposed to improve chemical conversion treatment property while the aim of increasing the strength by adding alloy elements being satisfied.
In the patent document 1, for example, a method to improve chemical conversion treatment property by controlling the regulation degree parameter representing the regularity of the roughness of the steel sheet surface to 0.25 or less is disclosed. The object of the control in this case is that of 340 MPa class or below belonging to a low-carbon Al-killed steel, and this technology scarcely exerts its effect for Mo-added steel which the present invention attends to in particular. Also, to secure a high strength steel sheet, utilization of alloy elements having a strengthening action such as Si and Mn becomes a useful measure. However, those alloy elements generate surface oxide in the annealing process after cold rolling, therefore chemical conversion treatment property cannot be improved by adjusting the regulation degree parameter of the roughness of the surface only, as far as the surface oxide is not controlled.
Also, in the patent document 2, lowering of chemical conversion treatment property is prevented by forming an iron coat of approximately 20-1,500 mg/m2 on the surface of the high strength cold rolled steel sheet and inhibiting the influence of the alloy element densified on the steel sheet surface and the selective oxidation layer. However, in this method, an electric plating process becomes necessary to form the iron coat, and problems of productivity and cost come up.
On the other hand, the present inventors developed a technology to effectively utilize the oxide generated on the steel sheet surface as a generating site of nuclei of phosphate crystal by controlling the shape of the oxide and to improve chemical conversion treatment property, and proposed previously as the patent document 3.    Patent document 1: JP-A-62-151208    Patent document 2: JP-A-5-320952    Patent document 3: JP-A-2005-187863